Proteasome inhibitors (Pis) such as Bortezomib (Btzmb), induce apoptosis in neoplastic cells through multiple mechanisms, including NF-KB activation, oxidative injury induction, an ER stress promotion, among others. Btzmb displays marked activity in multiple myeloma, and several types of non-Hodgkin's lymphoma (NHL), prompting the development of second generation proteasome inhibitors (e.g., PR-171) which may have superior PK/PD characteristics compared to Btzmb. However, the need to improve PI activity in NHL persists. Recent evidence from our laboratory suggests that combining Pis with other clinically relevant targeted agents, including HDAC or small molecule Bcl-2 inhibitors results in synergistic induction of apoptosis in NHL cell lines. The goal of this project is to elucidate, using a diffuse large B-cell lymphoma (DLBCL) model, mechanisms of synergism between PR-171 and these other targeted agents, extend these findings to an in vivo model system, identify laboratory-based response determinants, and use this information to initiate novel combination Phase I trials in patients with refractory NHL. In Specific Aim #1, we will determine whether synergism between Pis and HDAC inhibitors (vorinostat) in NHL cells reflects NF-KB disruption, induction of oxidative injury, ER stress, or a combination of these factors. In Specific Aim #2, we will determine whether synergism between Pis and small molecule Bcl-2 inhibitors (e.g. GX15-070) stems from oxidative injury, activation of stress-related signaling pathways, and/or Bak and Bax activation. We will also establish a) whether these approaches are effective in Btzmb-resistant DLBCL cells; and b) which of these individual strategies might be most appropriate for specific DLBCL sub-types i.e., germinal center (GC) versus activated B-cell (ABC). In Specific Aim #3, we will extend these findings to a xenograft DLBCL model system, and determine whether in vitro determinants of synergism are operative in vivo . In Specific Aim #4, we will select the most promising of regimens emanating from Aims #1-3 to pursue as one or more Phase I trials combining Pis with HDAC or small molecule Bcl-2 inhibitors in patients with refractory NHL. We will also conduct correlative laboratory studies to validate mechanisms of in vivo synergism, and identify surrogate markers of disease responsiveness. It is anticipated that these studies will lay the foundation for novel and potentially more effective Pi-based strategies in patients with refractory NHL.